As shown in JP-2006-183500A, a requested injection time that is necessary for a fuel injector to inject a fuel per one combustion cycle in an internal combustion engine is computed in response to the variables of an accelerator manipulated by a driver. The fuel injector is operated so as to keep the requested injection time and thereby a fuel of a quantity corresponding to an accelerator stroke is injected. In a spark ignition type internal combustion engine such as a gasoline engine for example, a requested intake air flow rate is computed in accordance with an accelerator stroke and a requested injection time is computed so that an air-fuel ratio most suitable for the requested intake air flow rate may be obtained.
Alcohol fuel is known as an alternate fuel for gasoline and light diesel oil (hereunder referred to as a regular fuel). When an alternate fuel is fed into a fuel tank in the state where a regular fuel remains therein to produce a composite fuel, the composite fuel is injected from the fuel injector and on this occasion the following problems arise.
It is known that, if it is attempted to obtain an excess air ratio equal to gasoline with an alcohol fuel for example, a larger amount of fuel injection than gasoline (for example about 1.6 times) is required. That is, with regard to the output torque of an internal combustion engine obtained by fuel injection for a certain period of time, the output torque obtained in the case of the injection of a composite fuel formed by blending alcohol with gasoline is smaller than that in the case of the injection of a fuel containing gasoline by 100%. Consequently, the injection time of a composite fuel must be longer than that of 100% gasoline.
The aforementioned requested injection time per one combustion cycle may exceeds the time allowing injection per one combustion cycle (720° CA) (an injectable time). In the operational region of a high output shaft rotation speed of an internal combustion engine in particular, the time required for one combustion cycle shortens, thus the injectable time also shortens, and resultantly the operation is likely to fall into the state where the requested injection time exceeds the injectable time.
When the requested injection time exceeds the injectable time, the air-fuel ratio deviates from an optimum value toward the side of a lean air-fuel ratio. In the case where the internal combustion engine is a gasoline engine in particular, if lean combustion occurs due to a lean air-fuel ratio, the combustion state is destabilized, thereby the amounts of HC and O2 flowing into a catalyst device increase, the HC and O2 are combusted in the catalyst device, and resultantly the temperature of the catalyst device rises and the catalyst device deteriorates.